First time smells are never forgotten

An Israeli researcher has discovered that first-time smells leave an indelible mark on the brain.

We all know that a smell can instantly transport us back to childhood or evoke pleasant or unpleasant memories, but now researchers in Israel believe they have found the reason why. It’s a discovery that could one day be put to use to help people to overcome psycho traumas.

In an experiment, scientists from the Weizmann Institute of Science in Rehovot led by graduate student Yaara Yeshurun tested a number of volunteers to see if their initial association of a smell with an experience would leave a unique and lasting impression on the brain.

In a special smell laboratory, volunteers were shown images of 60 visual objects – including chairs and pencils (objects that are not normally associated with any smells), and at the same time were presented with either a pleasant or an unpleasant odor including pear, fungus, and dead fish, generated by a machine called an olfactometer.

The subjects were then placed in a functional magnetic resonance imaging (fMRI) scanner and asked to recall the smells associated with each image. The entire test was repeated 90 minutes later with the same images but different odors.

Unique activity in the brain

A week later, Yeshurun and her team, Prof. Noam Sobel and Prof. Yadin Dudai, scanned the volunteers in the fMRI once more, to test which smell they most associated with each image shown.

The scientists discovered that after a week, even if the volunteer recalled both odors equally well (both the pleasant and unpleasant), the first association revealed noticeably activated levels of brain activity in the hippocampus and the amygdala, areas of the brain associated with memory, learning and emotion.

The effect was so clearly defined that the researchers were able to predict how well a volunteer recalled smells eight days later, based on their first fMRI scans. They also found that unpleasant odors made the biggest first impression.

The experiment was also conducted using sounds rather than smell, but the scientists discovered that sounds did not arouse a similar distinctive first-time pattern of activity. Their research was published in the latest issue of Current Biology.

Memory of bad smells makes evolutionary sense

“For some reason, the first association with smell gets etched into memory, and this phenomenon allowed us to predict what would be remembered one week later based on brain activity alone,” says Sobel.

“As far as we know, this phenomenon is unique to smell,” adds Yeshurun. “Childhood olfactory memories may be special not because childhood is special, but simply because those years may be the first time we associate something with an odor.”

In her research article, Yeshurun says it makes good sense to remember unpleasant memories as a kind of evolutionary “risk management”. “[It] may represent a potential adaptive mechanism considering the potential cost of failing to learn a first negative association and the potential benefit of a malleable first positive association,” she and her colleagues write.

In an article in ABC Science, Yeshurun says that any application of the findings is still far off, but suggests they could be used to help improve memories, or to help people erase early traumatic memories. “It may help us generate methods to better forget early and powerful memories, such as trauma,” she says.